1. Field of the Invention
The present invention relates to a sheet post-processing apparatus for an image forming apparatus, and specifically, relates to a sheet folding mechanism to perform a folding process which is utilized as one of functions of a sheet folding apparatus, a sheet folding apparatus using the same, and an image forming apparatus.
2. Description of the Related Art
A folding process is one of functions of a sheet post-processing apparatus. There has been known a technology of performing a folding process to bend a conveyed sheet by utilizing a sheet leading edge stopper, a flexure assist member and a folding roller and to wind the bent part to the folding roller.
To prevent occurrence of poor folding even when a curled sheet is conveyed, Japanese Patent Application Laid-open No. 2004-284719 discloses a structure including a first roller portion and a second roller portion which perform a folding process and a sheet guide member which is protruded toward the second folding roller portion at predetermined timing. Here, the sheet guide portion pushes an end part of a sheet-folded section formed by the first folding roller portion into a sheet nip portion during a folding process by the second folding roller portion.
A linear shape is popularly employed for a conveying path (i.e., a stopper conveying path) to a sheet leading edge stopper which constitutes a conventional sheet folding process. To satisfy market needs of product thinning, it is required that a stopper conveying path is formed to have a curved shape for performing a sheet folding process in a conserved space. In a case that the stopper conveying path is formed as being curved, in a mode of a two-time folding process for a sheet (e.g., inner threefold), the sheet once folded is conveyed to the stopper conveying path at the time of second folding. Then, the sheet forms a curvature when the sheet is abutted to the sheet leading edge stopper and is conveyed to the nip portion of folding rollers. The sheet is to be conveyed while the sheet edge at the upper side is abutted to a guide plate having the first fold line as a boundary during sheet conveyance. Then, there arises a problem of occurrence of box-folding, that is, another fold line is to be formed at a position being deviated from the first fold line when the second folding is performed as a result of occurrence of speed difference between an upper sheet and a lower sheet caused by the conveying load.
Therefore, there is a need for a mechanism to prevent occurrence of box-folding to have another fold line at a position being deviated from the first fold line when the second folding is performed.